Ideal place to do what? We still don't understand what you think you are trying to achieve. You seem to have switched from adding a capacitor in your HT supply, to adding an autotransformer in your bias supply. Neither makes any sense, unless we have seriously misunderstood you.

So now I'm thinking the ideal place to do this is in the secondary that feeds the bias supply rectifier. I can just use a small autoformer in the bias supply secondary to lower the bias supply AC voltage level the small variation I need without having to desolder and replace 10 bias trim pots X 2 amps.

Hmm....

I think you NEED to show us a schematic of your amp As It Currently Is...

And a schematic of your amp With Your Proposed Modification....

Your amp has Individual Bias Pots FOR A REASON!
--This is to Match Each Valve to its neighbour. Valves are Never exactly the same--Even so-called, Matched valves. All AGE differently!

What are you trying to achieve...?
--A master bias pot to set them all....??

I Seriously Suspect you could cause serious damage either to the Valves or other parts of the amplifier if you start messing round with the Bias Supply by shoving a cap in series with it!
--You HAVE TO have a DC Voltage for Neg. Bias, or the valves will soon self-destruct!

Please Please, Before you do Anything Revise your knowledge of AC and DC cuurents in circuits--Particularly DC!
--I'm not being funny, just trying to save you what could well be a VERY EXPENSIVE mistake!

Because tubes pass only DC. No DC, no tube amp. The DC is modulated into pulses, but it's always only passing current in that same single direction. Then the DC "bias" "component" or whatever you want to call it is "blocked" so only the "pulses" are presented to the speaker or the next tube's input.

Caps block DC when they are inline and they "smooth" DC when they are parallel to the DC. If you place one inline with the B+ you block it and prevent the related tubes from functioning.

This is not the place to argue about what is meant by "DC". There has already been a long thread about that. Valves are unidirectional, which is not necessarily the same as DC. It makes more sense to define DC as meaning frequencies around 0Hz in a Fourier analysis, except for elementary textbooks for newbies.